Summary:
Option `-fskip-odr-check-in-gmf` is set by default and I think it is what most of C++ developers want. But in header units clang ODR checking is too strict that makes them hard to use, see example in the diff. This diff relaxes ODR checks for unnamed modules to match GMF ODR checking.

Alternative solution is to add special handling for CXXFoldExpr to ignore differences in “callee”. In particular it will treat the following fragments identical. I think it might be reasonable default behavior instead of `-fskip-odr-check-in-gmf`.
```
CXXFoldExpr 0x55556588b1b8 '<dependent type>'
|-<<<NULL>>>
|-UnresolvedLookupExpr 0x55556588b140 '<dependent type>' lvalue (no ADL) = '__cmp_cat_id' 0x55556588ac18
| `-TemplateArgument type '_Ts'
|   `-TemplateTypeParmType 0x55556588adf0 '_Ts' dependent contains_unexpanded_pack depth 0 index 0 pack
|     `-TemplateTypeParm 0x55556588ad70 '_Ts'
`-<<<NULL>>>

CXXFoldExpr 0x55556588b670 '<dependent type>'
|-UnresolvedLookupExpr 0x55556588b628 '<overloaded function type>' lvalue (ADL) = 'operator|' 0x55556588ae60
|-UnresolvedLookupExpr 0x55556588b5b0 '<dependent type>' lvalue (no ADL) = '__cmp_cat_id' 0x55556588b0d8
| `-TemplateArgument type '_Ts'
|   `-TemplateTypeParmType 0x55556588b260 '_Ts' dependent contains_unexpanded_pack depth 0 index 0 pack
|     `-TemplateTypeParm 0x55556588b1e0 '_Ts'
`-<<<NULL>>>
```

Test Plan: check-clang